This invention relates in general to electrophotography and, more specifically, to a novel electrophotographic imaging member and process for using the imaging member.
In the art of electrophotography, an electrophotographic imaging member containing a photoconductive layer is imaged by first uniformly electrostatically charging the imaging surface of the imaging member. The member is then exposed to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the photoconductive layer while leaving behind an electrostatic latent image in the non-illuminated areas. The electrostatic latent image may then be developed to form a visible image by depositing finely divided properly charged toner particles on the surface of the photoconductive layer to form a toner image which is thereafter transferred to a receiving member and fixed thereto.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite of layers containing a photoconductor and another material. One type of composite photoconductive photoreceptor used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two elecrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Such a photoconductive layer is often referred to as a charge generating or photogenerating layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with uniform charges of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may function as a cathode when the charge transport layer is sandwiched between the electrode and a photoconductive layer which is capable of photogenerating holes and electrons and injecting the photogenerated holes into a charge transport layer when the outer surface of the photoconductive layer is charged with uniform charges of a positive polarity.
Other types of composite photoconductor employed in xerography include photoresponsive devices in which a conductive substrate or electrode is coated with optional blocking and/or adhesive layers, a charge transport layer such as a hole transport layer, and a photoconductive layer. Where the transport layer is a hole transport layer, the outer surface of the photoconductive layer is charged positively. These types of composite photoconductors are described, for example, in copending applications U.S. Ser. No. 613,137, filed on May 23, 1984, entitled "Silylated Compositions and Deuterated Hydroxyl Squaraine Compositions and Processes" and U.S. Ser. No. 487,935, filed on Apr. 25, 1983, entitled "Overcoated Photoresponsive Devices", the entire disclosures thereof being incorporated herein in their entirety.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical inorganic photoconductive materials utilized in the charge generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The organic photoconductive materials utilized in the charge generating layer include metal free phthalocyanines, vanadyl phthalocyanines, substituted and unsubstituted squaraine compounds, thiopyrylium compounds and azo and diazo dyes and pigments. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Some examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990, the disclosure of this patent being incorporated herein in its entirety.
Organic photoreceptors can comprise either a single layer or a multilayer structure. The commonly used multilayered or composite structure contains at least a photogeneration layer, a charge transport layer and a conductive substrate. The photogeneration layer generally contains a photoconductive pigment and a polymeric binder. The charge transport layer (e.g. hole transport layer) contains a polymeric binder and charge transport molecules (e.g., aromatic amines, hydrazone derivatives, etc.). These organic, low ionization potential hole transport molecules as well as the polymeric binders are very sensitive to oxidative conditions arising from photochemical, electrochemical and chemical reactions. In copiers, duplicators and electronic printers they are frequently exposed to hazardous environmental conditions which include light, charging devices such as corotrons, dicorotrons, scorotrons and the like, electric fields, oxygen, oxidants and moisture. Undesirable chemical species are often formed during fabrication or during use in imaging processes which may react with key organic components in the charge transport layer or photogeneration layer of the photoreceptors. These unwanted chemical reactions can cause photoreceptor degradation, poor charge acceptance and cyclic instability.
Several types of reactive chemical species that are likely to be formed in the operational environment of a copier or an electronic printer include:
(a) Oxidants (e.g. peroxides, hydroperoxides, ozone, oxygen, selenium, selenium oxide, selenium alloys, arsenic oxide, vanadium oxide, VOPc, and the like) may vary depending on the type of photoreceptor used. PA0 (b) Both organic and inorganic radicals and diradicals (e.g. R.sup..multidot., RO.sub.2.sup..multidot., O.sub.2.sup..multidot., NO.sub.2.sup..multidot., OH.sup..multidot., and the like.). PA0 (c) Ionic species having positive (e.g. aromatic amine+.sup..multidot.) or negative (e.g. O.sup.-) charges. PA0 (d) Both singlet oxygen states (i.e. .sup.1 O.sub.2 (Sigma.sup.+ g) and .sup.1 O.sub.2 (.DELTA.g) can form through a sensitized photooxidation mechanism. PA0 (1) Antioxidants for autooxidation (free radical inhibitors or quenchers or stabilizers) which can prevent or retard the autooxidation of organic material including aromatic diamine charge transport molecules, aromatic amine derivatives and hydrazone compounds. These may also hinder the formation of undesired conductive species in the photoreceptor with or without the presence of light. Many known classes of antioxidants are include: (a) amine derivatives (e.g., aliphatic, cyclic, aromatic and heterocyclic amines), (b) phenolic derivatives (e.g., substituted phenols, butylated hydroxytoluene(2,6-di-t-butyl-p-cresol), butylated hydroxyanisole(2,6-di-t-butyl-4-methoxyphenol and 2-t-butyl-4-methoxyphenol), 2,6-di-t-butylphenol, alpha-tocopherol, other tocopherol related materials, 2,3,6-triphenylphenol, pentaerythrityl tetrakis[beta-(4-hydroxy,3,5-di-t-butylphenyl)proprionate] (Irganox 1010), and the like, (c) sulfides: disulfides, thiodipropionate esters and thiols, (d) aryl phosphites, and (e) organo tin and boron compounds. PA0 (2) Antioxidants for the inhibition of sensitized photooxidation involving singlet oxygen. Antioxidants such as singlet oxygen quenchers involve a wide variety of useful materials. These include various hydroxybenzene derivatives such as 2,6-di-t-butylphenol, butylated hydroxytoluene, butylated hydroxyanisole, alpha-tocopherol, tocopherol derivatives. Irganox 1010, 2,4,6-triphenylphenol, hydroquinones and phenolic polymers. Hindered substituted phenols are particularly effective.
The foregoing chemical species can be generated from chemical, electrochemical and photochemical reactions as well as from the corona discharge in air by a charging device. The oxidative intermediates and their products usually degrade the photoreceptor and lead to various electrical problems. If the entire photoreceptor degrades as a result of chemical and photchemical reactions, the photoreceptor becomes conductive (e.g. develops high dark decay) and exhibits poor charge acceptance, aging and stability deficiencies. Depending on the degree of damage, the photoreceptor degradation can lead to poor image quality, cycle-up, and cycle-down problems or even an inability of a copier or an electronic printer to produce a print.
Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 and copending applications U.S. Ser. No. 613,137, filed on May 23, 1984, entitled "Silylated Compositions and Deuterated Hydroxyl Squaraine Compositions and Processes" and U.S. Ser. No. 487,935, filed on Apr. 25, 1983, entitled "Overcoated Photoresponsive Devices", provide excellent images when charged with a uniform electrostatic charge, exposed to a light image and thereafter developed with finely divided toner particles. However, when the charge transport layer comprises a film forming resin and one or more of certain aromatic amines, diamines and hydrazone compounds, difficulties have been encountered with these photosensitive members when they are used under certain conditions in copiers, duplicators and printers. For example, one undesirable effect occurs when the use of corona devices causes an increased degradation rate of the photoreceptor or when the photosensitive members are exposed to ultraviolet (U.V.) radiation. In other words, the charge acceptance capability of the photosensitive member decreases upon exposure to U.V. radiation or in the presence of free radicals. Moreover, multilayered photoreceptor devices utilizing either a transport layer sandwiched between a conductive layer and a photogeneration layer or a photogeneration layer sandwiched between a conductive layer and transport layer exhibit an increased dark decay rate under the adverse influence of corona charging devices. Dark decay is defined as the loss of charge on a photoreceptor in the dark after uniform charging. This is an undesirable fatigue-like problem resulting in lower initial charges and contrast potential that cannot be properly maintained during image cycling and is unacceptable for automatic electrophotographic copiers, duplicators and printers which require precise, stable, and a predictable photoreceptor operating range. More specifically, such devices generally experience low charge acceptance rates in the first few imaging cycles. The charge acceptance level gradually increases (cycle-up) upon cycling and eventually reaches an almost constant value. The poor initial charge acceptance of a photoreceptor causes poor image quality, light image density, poor solid area density or image deletion in the first few xerographic copies. This problem becomes more serious if the photoreceptor has been used for some time and dark-rested for several hours (e.g. overnight). For example, the charge acceptance level after dark-resting for photoreceptors containing vanadyl phthalocyanine in the charge generating layer (often referred to as a photogenerating or photoconductive layer) is normally lower than it normally would be under conditions where it has not dark-rested for several hours. This problem also occurs in other photoreceptors not containing vanadyl phthalocyanine and causes poor image quality of a printed copy and is considered a long-term cycle induced reduction of charge acceptance. If machine adjustments to compensate for these changing properties are made, copies made layer during cycling exhibit high background and poor images. The severity of the problem appears to be proportional to the number of copies made immediately preceeding shut down and also to the length of time the system is allowed to remain at rest. In the worst situation, the photoreceptor cannot be charged at all and is totally useless.
Thus, the characteristics of photosensitive members comprising a conductive layer and at least two electrically operative layers, one of which is a charge transport layer comprising a film forming resin and one or more aromatic amine compounds or hydrazone derivatives, exhibit deficiencies which are undesirable in modern copiers, duplicators, and printers. Accordingly, there is a need for compositions and processes which impart greater stability to electrophotographic imaging systems which undergo periodic cycling.